Chlorofluorocarbons (CFCs, i.e., compounds containing only carbon, fluorine and chlorine) have been used for many years as refrigerants, heat transfer media, foam expansion agents, aerosol propellants, solvents and power cycle working fluids. For example, various CFC solvents have been used as cleaning liquids for the removal of contaminants from contaminated articles and materials. Certain fluorine-containing organic compounds such as 1,1,2-trichloro-1,2,2-trifluoroethane (CFC-113) have been reported as useful for this purpose, particularly with regard to cleaning organic polymers and plastics which may be sensitive to other more common and more powerful solvents such as trichloroethylene or perchloroethylene. Recently, however, there have been efforts to reduce the use of certain chlorofluorocarbon compounds such as CFC-113 because of a concern over their potential to deplete ozone, and to thereby affect the layer of ozone that is considered important in protecting the earth's surface from ultraviolet radiation. Consequently, there is a worldwide effort to find alternative compounds which contain fewer or preferably no chlorine substituents.
Boiling point, flammability and solvent power can often be adjusted by preparing mixtures of solvents. For example, certain mixtures of 1,1,2-trichloro-1,2,2-trifluoroethane with other solvents (e.g., isopropanol and nitromethane) have been reported as useful in removing contaminants which are not removed by 1,1,2-trichloro-1,2,2-trifluoroethane alone, and in cleaning articles such as electronic circuit boards where the requirements for a cleaning solvent are relatively stringent (i.e., it is generally desirable in circuit board cleaning to use solvents which have low boiling points, are non-flammable, have low toxicity, and have high solvent power so that flux such as rosin and flux residues which result from soldering electronic components to the circuit board can be removed without damage to the circuit board substrate).
While boiling point, flammability, and solvent power can often be adjusted by preparing mixtures of solvents, the utility of the resulting mixtures can be limited for certain applications because the mixtures fractionate to an undesirable degree during use. Mixtures can also fractionate during recovery, making it more difficult to recover a solvent mixture with the original composition. Azeotropic compositions, with their constant boiling and constant composition characteristics, are thus considered particularly useful.
The properties of halogenated hydrocarbons can be influenced by the arrangement of the halogens (and hydrogen, when present) on the carbon framework. One of the challenges in preparing compounds containing fluorine and hydrogen has been achieving the desired arrangement of such substituents.
One arrangement involves providing a hydrogen on different carbons spaced a selected distance from one another along a carbon chain. For example, it can be desirable to provide a hydrogen substituent on each of two carbon atoms which are separated from one another by a chain of two other carbon atoms. 1,1,2,2,3,3,4,4-Octafluorobutane (HFC-338pcc) is such a compound. HFC-338pcc forms useful blends, and particularly azeotropes, with solvents such as alcohols, ketones, and other halogenated solvents to form compositions useful for cleaning surfaces, especially electronic components as disclosed in U.S. Pat. No. 5,250,208, U.S. Pat. No. 5,221,493, and U.S. Pat. No. 5,194,170. There is a need for non-chlorinated solvents like HFC-338pcc (which have little effect on the ozone layer) as replacements for more chlorinated solvents such as CFC-113. There is also a need for processes for effectively producing compounds such as HFC-338pcc.
U.S. patent application Ser. No. 08/315,025 (the priority document for PCT International Publication No. WO 96/10002), which is incorporated herein by reference, discloses a process for the preparation of product compounds of the formula HC(R.sup.1).sub.2 C(R.sup.1).sub.2 C(R.sup.2).sub.2 C(R.sup.2).sub.2 H (e.g., CHF.sub.2 CF.sub.2 CF.sub.2 CHF.sub.2, HFC-338pcc) by reaction of a metallacycle of the formula ##STR1##
with hydrogen.